The primary components of a motor include stator/winding assembly, rotor/shaft/bearing assembly, and two end caps. Some specific motor applications include their use in printers, security cameras, x-y tables, scanners, CNC machines, dispensers, injector pumps, turntables, optical equipment, and any precision devices of motion control system. Many of these applications either require or work better with a small motor size, including small dimensions measured from the mounting face to the rear of the motor.
A traditional motor provides mounting threads (often four in number) in the front end cap for receiving screws extending from the face of whatever the motor is mounted to. However, using small/short screws to mount the motor becomes ever more difficult as the device as gets smaller and more compact, as the screws normally cannot extend further than the thickness of the end cap into the motor itself without interfering with the operation of some functional part of the motor.
A rear mounting configuration is an alternative seen in many motor applications. Instead of using short screws to hold the motor on a plate from the front, much longer screws extend from the motor's rear into threads of a mounting plate in order to secure the motor to the plate. In that variation, the rear end cap extends outward beyond the perimeter of the stator in order to accommodate apertures for passage of the long screws.
In many applications, dissipation of heat generated by the motor is an important requirement. Many possible structural adaptations to facilitate heat dissipation tend to increase overall motor size, and therefore are often a primary cause of losses in overall performance for a given motor size. To minimize the size increase, engineers typically design a motor's stator with a circular or square shape. Generally, one doesn't see any other shape besides those two. Accordingly, with such a stator shape, the majority of heat dissipation would have to proceed through the front end cap into the mounting plate.